Of interest is Ser. No. 127,335 filed Dec. 1, 1987 entitled "DPCM SYSTEM WITH ADAPTIVE QUANTIZER HAVING UNCHANGING BIN NUMBER ENSEMBLE" by A. A. Acampora, Ser. No. 130,379 filed Dec. 8, 1987 entitled "MODIFIED STATISTICAL CODING OF DIGITAL SIGNALS" by N. J. Fedele and Ser. No. 158,171 filed Feb. 19, 1988 entitled "DECODER FOR DIGITAL SIGNAL CODES" by R. Hingorani et al. all assigned to the assignee of the present invention.
Differential pulse code modulation (DPCM) is an often-used step in data compression of digital signals representative of raster-scanned sampling of sequences of image frames. Further data compression can be obtained by subjecting the differential pulse code modulation signals to a structural coding procedure, such as the run length encoding of runs of successive zero-valued samples. Still further data compression can be obtained by a statistical coding procedure where variable length codes are assigned to various differential pulse code values or structural codes, with shorter length codes being assigned to more frequently encountered conditions and longer length codes being assigned to less frequently occurring conditions. Statistical coding is generally carried out using a look-up table, wholly or partially stored in read-only memory (ROM) or its like.
In the prior art, the statistic encoding of DPCM signals and the statistical coding of mixtures of DPCM signals and structural codes derived therefrom has generally presumed independence of events in the acquiring of statistics concerning the frequency of those events. A single look-up table based on these statistics is used for coding.
In digital image transmission systems using DPCM, however, running statistics can be kept over substantially large numbers of consecutive samples (which procedure is sometimes referred to as histogram generation) and the entries in the look-up table can be modified from time to time reflective of change in the statistics concerning the frequency of events. To implement this procedure, a library of look-up tables for statistical coding can be kept, one of which is selected at a time, selection being based on best fit to the statistics of the signal as determined from large numbers of consecutive samples. These procedures find their intellectual basis for being used in the fact that only most of the time are there good degrees of correlation between spatially adjacent samples in the case of intraframe DPCM and between corresponding samples in successive frames in the case of interframe DPCM. Such correlation when it occurs reduces the populations of higher-value DPCM samples relative to the populations of lower-value DPCM samples, when a large number of samples are considered. Having alternative coding tables available helps accommodate conditions where such correlations do not obtain--e.g. when there is a lot of motion in the sequence of image frames being digitally transmitted. The foregoing procedures were known to some highly skilled in the image encoding art prior to the invention to be described.
The present invention does not rely on correlation between the absolute values of image samples on a pairwise basis for selecting among different statistical coding procedures. Rather the invention relies on the effects upon DPCM statistics caused by restrictions on dynamic range in the descriptions of how each of the samples differs from another sample used to predict its value. One restriction on dynamic range is imposed by the digital channel capacity in terms of bits per sample. Other restrictions may be imposed by slew rate limitations on the original video signals that are to be digitally transmitted, i.e., rate of change of adjacent pixel values. In intraframe DPCM where samples are predicted from respective just-preceding samples in the same scan line such slew rate limitations tend to be imposed on digital video signals derived from analog video signals. In interframe DPCM where samples are predicted from samples in the preceding scan line, such slew rate limitations will not be found absent transversed filtering steps being taken prior to differential pulse code modulation taking place. In interframe DPCM unless temporal filtering procedures of frame-averaging nature are followed, slew rate limitations will not be found between samples in one image frame and correspondingly located samples in the preceding image frame.
The invention takes advantage of the fact that information concerning a previous sample used for prediction in a DPCM transmission system permits elimination of certain ranges of difference between that previous sample and the current sample in a DPCM coding procedure, so that the number of bits available per DPCM code sample may be more efficiently allocated to describe the possible ranges of difference. That is, binning in the DPCM quantizer can be made to describe finer variations in the difference for a given number of bits per DPCM code sample. At the same time, a different set of statistical codes can be selected for statistically encoding the DPCM (as may be modified by run-length encoding of zero-value DPCM samples).